The present invention relates in general to integrated circuits and, more particularly, to a color balancing circuit for processing imaging signals.
High-resolution image capturing systems typically capture an image by projecting light from the image onto an array of semiconductor photoactive sensing devices such as charge-coupled devices, p-i-n diodes or phototransistors. Such arrays operate at low power and are easily integrated on a semiconductor die, and so are ideal for use in portable equipment such as digital cameras and scanners. Each photoactive sensing device converts the light to a pixel signal. The system output provides image data containing intensity and color information for each pixel signal in a format displayable by a display device.
Image colors are separated into three or four component primary colors by interposing a matrix of optical color filters between the image and the sensing array such that light of substantially one primary color reaches each photoactive device. For example, a system based on red, green and blue primary colors is commonly known as an RGB system which uses a matrix of red, green and blue color filters. Hence, each photoactive device senses light of one primary color to produce a pixel signal representing the primary color as well as the intensity of the light. The primary colors are blended by the display device to reproduce the other colors of the image. Where the display device is a high definition television, pixel signals are generated at a rate of at least 27.0 megahertz.
On most systems, the response of photoactive devices to their respective primary colors is unbalanced, often due to the color filters. For example, devices that sense red light may produce pixel signals of greater amplitude than devices sensing blue light of the same intensity. If the color imbalance is not corrected, the image is not displayed with accurate colors.
To correct for the differences in color sensitivity, pixel signals of different primary colors are amplified by different gains to color balance the pixel signals. Prior art systems perform such color balancing by amplifying the pixel signals of each primary color in a different amplifier set to the appropriate compensating gain. For example, to color balance a red pixel signal generated by red light and having twice the amplitude of a blue pixel signal generated by blue light of equal intensity, the blue pixel signal is amplified in one amplifier having twice the gain as another amplifier that amplifies the red signal. Such multiple amplifier color balancing suffers from image display artifacts such as fixed pattern noise due to mismatches among the amplifiers. Such artifacts are difficult or impossible to identify and correct.
Hence, there is a need for an improved circuit and method of color balancing pixel signals to compensate for differences in the color response of photoactive sensing devices.